We have learned in the past few weeks that salt water is more dense than fresh water and that cold water is more dense than warm. How do the factors of salinity and temperature combine?
|The two jars react very differently when the colored ice cubes are added to them.|
We have learned in the past few weeks that salt water is more dense than fresh water and that cold water is more dense than warm. How do these factors, salinity and temperature, combine?
Before hand, make some colored ice cubes by adding food coloring to the water in an ice cube tray. We did this the night before.
Fill two identical jars about 3/4 full with tap water. Add about 1/4 cup Kosher salt to one of the jars, mix thoroughly and let sit. Can your students tell which one of them has salt water and one fresh water (without tasting)? Very carefully add 3-4 of the colored ice cubes to each jar. Add the same amount to each jar. Do not bump or disturb the jars.
Where is the colored water going? Which jar is the ice melting faster? What would this indicate?
Can you see a current flowing toward the bottom as the icy water carries the food coloring down with it as it sinks? As the temperature evens out, the food coloring mixes throughout the jar. This is the fresh water jar. The icy water is denser than the room temperature water and sinks. The sinking of the icy water helped set up currents in the jar which quickly mixed the icy water and the room temperature water.
In the other jar the ice melted more slowly and the food coloring formed a layer at the surface. Because the ice cubes were made of fresh water, as they melted, the water floated at the top of the denser salt water.
Originally posted Feb 11, 2011